Finite element analysis of convective heat transfer in porous media

The finite element method is used to analyse convective heat transfer in a porous medium. Convection past a vertical surface embedded in the medium and convection in a confined porous medium enclosure are analysed using the above method. The results are compared with those available in the literature and the agreement is found to be good. The method is applicable for two-dimensional analysis in a porous body of any arbitrary shape. The restriction of the boundary layer assumption is relaxed.     

Hydraulic and heat transfer study of SiO2/water nanofluids in horizontal tubes with imposed wall temperature boundary conditions

The convective heat transfer of SiO₂/water colloidal suspensions (5-34 wt.%) is investigated experimentally in a flow loop with a horizontal tube test section whose wall temperature is imposed. Experiments were performed at different inlet temperatures (20, 50, 70 °C) in cooling and/or heating conditions at various flow rates (200 < Re < 10,000). The Reynolds and Nusselt numbers were deduced by using thermal conductivity and viscosity values measured with the same temperature conditions as those in the tests. Results indicate that the heat transfer coefficient values are increased from 10% to 60% compared to those of pure water. They also show that the general trend of standard correlations is respected. The problem of suspension stability at the highest temperatures is discussed. In order to evaluate the benefits provided by the enhanced properties of the nanofluids studied, an energetic performance evaluation criterion (PEC) is defined. This PEC decreases as the nanoparticle concentration is increased. This process is also discussed in this paper.     

Numerical experiments on convective heat transfer in water-saturated porous media at near-critical conditions

Fluid and heat flow at temperatures approaching or exceeding that at the critical point (374 °C for pure water, higher for saline fluids) may be encountered in deep zones of geothermal systems and above cooling intrusives. In the vicinity of the critical point the density and internal energy of fluids show very strong variations for small temperature and pressure changes. This suggests that convective heat transfer from thermal buoyancy flow would be strongly enhanced at near-critical conditions. This has been confirmed in laboratory experiments. We have developed special numerical techniques for modeling porous flow at near-critical conditions, which can handle the extreme nonlinearities in water properties near the critical point. Our numerical simulations show strong enhancements of convective heat transfer at near-critical conditions; however, the heat transfer rates obtained in the simulations are considerably smaller than data reported from laboratory experiments by Dunn and Hardee. We discuss possible reasons for this discrepancy and develop suggestions for additional laboratory experiments.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Convection and heat/mass transfer processes under space flight conditions

A simulation of 3D convective flows and heat/mass transfer processes under space flight conditions on the basis of hydrodynamic models and a numerical analysis of these models is discussed. The significance of the methods of mechanics in microgravity sciences and the role of the journal “Fluid Dynamics” in the development of this branch of science is examined. The results of recent investigations of certain problems are presented.     

基于COMSOL的简化对流传热结构拓扑优化方法

提出了一种基于COMSOL平台的简化对流传热结构拓扑优化实现方法。简化对流传热拓扑优化可用于获得性能更优的散热结构概念设计,但缺乏可行性强和适用范围广的实现方法,导致工程应用存在较高的技术门槛。为解决该问题,本研究采用在经典传热方程中引入对流换热项的方式,建立了可用于COMSOL平台简化对流换热分析的控制方程。进一步的,结合COMSOL平台提供的传热分析模块以及拓扑优化模块,搭建了简化对流传热结构拓扑优化计算框架。数值算例考虑了经典二维和三维的简化对流传热结构优化设计问题。结果显示了本文方法的有效性及可行性。     

Flow and heat transfer in a rotating cavity with a radial inflow of fluid Part 1: The flow structure

Flow visualization has been conducted in a rotating cavity, comprising two steel discs and a peripheral polycarbonate shroud, for dimensionless flow rates of air up to |C_w|8000 and rotational Reynolds number up to Re_φ10⁶. For all the experiments, the ratio of the inner to outer radii of the discs was 0.1 and the ratio of the axial clearance between the discs to their outer radii was 0.133; five different shroud geometries were tested. The flow visualization has confirmed that the flow structure comprises a source region near the shroud, laminar or turbulent Ekman layers on the discs, a sink layer near the centre of the cavity, and an interior core of rotating fluid. Above a certain flow rate, this structure was found to be unstable; heating one disc tended to stabilize the flow. For isothermal flow, measurements of the size of the source region were in good agreement with values predicted from a simple theoretical model.     

Flow and heat transfer in a second grade fluid over a stretching sheet

An analysis is carried out to study the flow and heat transfer characteristics in a second grade fluid over a stretching sheet with prescribed surface temperature including the effects of frictional heating, internal heat generation or absorption, and work due to deformation. In order to solve the fourth-order non-linear differential equation, associated with the flow problem, a fourth boundary condition is augmented and a proper sign for the normal stress modulus is used. It is observed that for a physical flow problem the solution is unique. The solutions for the temperature and the heat transfer characteristics are obtained numerically and presented by a table and graphs. Furthermore, it is shown that the heat flow is always from the stretching sheet to the fluid.     

Convective heat transfer characteristics of high-pressure gas in heat exchanger with membrane helical coils and membrane serpentine tubes

Since the heat transfer performance of syngas cooler affects the efficiency of the power generating system with integrated coal gasification combined cycle (IGCC) directly, it is important to obtain the heat transfer characteristics of high-pressure syngas in the cooler. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is experimentally investigated. High pressure single gas (He or N₂) and their mixture (He + N₂) gas serve as the test media in the test pressure range from 0.5 MPa to 3.0 MPa. The results show that the convection heat transfer coefficient of high pressure gas is influenced by the working pressure, gas composition and symmetry of flow around the coil, of which the working pressure is the most significant factor. The average convection heat transfer coefficients for various gases in heat exchangers are systematically analyzed, and the correlations between Nu and Re for two kinds of membrane heat exchangers are obtained. The heat transfer coefficient of heat exchanger with membrane helical coils is greater than that of the membrane serpentine-tube heat exchanger under the same conditions. The heat transfer coefficient increment of the membrane helical-coil heat exchanger is greater than that of the membrane serpentine-tube heat exchanger with the increase of gas pressure and velocity.     

Convective heat transfer of nanofluids with correlations

To investigate the convective heat transfer of nanofluids, experiments were performed using silver–water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver–water nanofluid, with ±10% agreement between experiments and prediction.     

Convective heat transfer in the laminar–turbulent transition region with molten salt in a circular tube

In order to understand the heat transfer characteristics of molten salt and testify the validity of the well-known empirical convective heat transfer correlations, experimental study on transition convective heat transfer with molten salt in a circular tube was conducted. Molten salt circulations were realized and operated in a specially designed system over 1000 h. The average forced convective heat transfer coefficients of molten salt were determined by least-squares method based on the measured data of flow rates and temperatures. Finally, a heat transfer correlation of transition flow with molten salt in a circular tube was obtained and good agreement was observed between the experimental data of molten salt and the well-known correlations presented by Hausen and Gnielinski, respectively.     

Flow and heat transfer with prescribed skin friction in a rotating fluid

This paper investigates the rotating flow and heat transfer of a viscous fluid induced by a stretching surface. The nonlinear problem subject to a given skin friction at the boundary is solved. Analytic solution is obtained using homotopy analysis method. The velocity, temperature, and stretching velocity is calculated for different values of the rotation parameter (λ). The obtained results are compared with the well known results of rotating flow induced by a stretching surface by using four sets of boundary conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Review of nanofluids for heat transfer applications

Research on nanofluids has progressed rapidly since its enhanced thermal conductivity was first repotted about a decade ago,though much controversy and inconsistency have been reported,and insufficient understanding of the formulation and mechanism of nanofluids further limits their applications.This work presents a critical review of research on heat transfer applications of nanofluids with the aim of identifying the limiting factors so as to push forward their further development.     

Flow and heat transfer over a backward-facing step with a cylinder mounted near its top corner

A study was made to see if it is possible to enhance the heat transfer in the downstream region of a backward-facing step, where heat transfer is normally deteriorated, by the insertion of a cylinder near the top corner of the step. Cylinder size and streamwise position of the cylinder were kept constant but the cross-stream position of the cylinder was changed in three steps. Results of the heat transfer experiment, flow visualization, and measurement of the averaged and fluctuating flow fields were reported. When the cylinder was mounted at a position, a little higher than the top surface of the step, a jet-like flow pattern emerged in the averaged velocity profile beneath the cylinder and the recirculating flow was intensified. Therefore, the velocity of recirculating flow near the wall is increased at some streamwise positions. Additionally, the velocity fluctuation was intensified not only in the shear layer between the jet-like flow and the recirculating flow regions but also in the near wall region, resulting in the effective augmentation of heat transfer in this case. Therefore, it is concluded that the mounting of a cylinder is effective in the enhancement of deteriorated heat transfer in the recirculating flow region, if its is mounted in a proper position.     

Measurement and correlation of volumetric heat transfer coefficients of cellular ceramics

The volumetric heat transfer coefficients (h_v) between cellular ceramics and a stream of air were measured using the single-blow transient experimental technique in conjunction with an inverse analysis. Test specimen made of mullite, YZA, SiC, cordierite and cordierite with LS-2 coating was studied. The number of pores per centimeter (PPC) ranged from 4 to 26 and the specimen thickness ranged from 6 to 12 mm. Based on the experimental data, the volumetric heat transfer coefficients were generalized by developing Nusselt number vs. Reynolds number correlations of the form Nu_v=C Re^m for the materials studied. The effects of pore length-scale and specimen thickness on the volumetric heat transfer coefficients are presented and discussed.     

Flow boiling heat transfer characteristics of R123 and R134a in a micro-channel

Flow boiling heat transfer in a single circular micro-channel of 0.19 mm ID has been experimentally investigated with R123 and R134a for various experimental conditions: heat fluxes (10, 15, 20 kW/m²), mass velocities (314, 392, 470 kg/m² s), vapor qualities (0.2–0.85) and different saturation pressures (158, 208 kPa for R123; 900, 1100 kPa for R134a). The heat transfer trends between R123 and R134a are clearly distinguished. Whether nucleate boiling is suppressed at low vapor quality or not determines the heat transfer trend and mechanism in the flow boiling of micro-channels. High convective heat transfer coefficients in the two-phase flow of micro-channels enables nucleate boiling to be suppressed even at low vapor quality, depending on the wall superheat requirement for nucleate boiling. In the case of early suppression of nucleate boiling, specifically R123, heat transfer is dominated by evaporation of thin liquid films around elongated bubbles. In the contrary case, namely R134a, nucleate boiling is dominant heat transfer mechanism until its suppression at high vapor quality and then two-phase forced convection heat transfer becomes dominant. It is similar to the heat transfer characteristic of macro-channels except the enhancement of nucleate boiling and the short forced convection region.     

A new numerical scheme for convective dominated heat transfer in a porous medium with strong temperature gradients

A new numerical scheme, theimplicit correction scheme, has been developed for heat transfer in a porous medium with strong temperature gradients. The scheme includes diffusion, convection and transverse heat transfer processes. By using correction coefficients which are based on transverse heat transfer, the effects of convection enthalpy flow and diffusion are modified. Under suitable limiting conditions, the implicit correction scheme can be reduced to the central-difference, upwind, or power-law scheme. The correction scheme is shown to be especially useful in calculations of the thermal effectiveness of the regenerator in Stirling cycle refrigeration.     

A tube-by-tube reduction method for simultaneous heat and mass transfer characteristics for plain fin-and-tube heat exchangers in dehumidifying conditions

This study proposed a new method, namely a tube-by-tube reduction method to analyze the performance of fin-and-tube heat exchangers having plain fin configuration under dehumidifying conditions. The mass transfer coefficients which seldom reported in the open literature, are also presented. For fully wet conditions, it is found that the reduced results for both sensible heat transfer performance and the mass transfer performance by the present method are insensitive to change of inlet humidity. Unlike those tested in fully dry condition, the sensible heat transfer performance under dehumidification is comparatively independent of fin pitch. The ratio of the heat transfer characteristic to mass transfer characteristic (h_c,o/h_d,o C_p,a) is in the range of 0.6~1.0, and the ratio is insensitive to change of fin spacing at low Reynolds number. However, a slight drop of the ratio of (h_c,o/h_d,o C_p,a) is seen with the decrease of fin spacing when the Reynolds number is sufficient high. This is associated with the more pronounced influence due to condensate removal by the vapor shear. Correlations are proposed to describe the heat and mass performance for the present plate fin configurations. These correlations can describe 89% of the Chilton Colburn j-factor of the heat transfer (j_h) within 15% and can correlate 81% of the Chilton Colburn j-factor of the mass transfer (j_m) within 20%.     

Flow boiling heat transfer of R134a in the multiport minichannel heat exchangers

The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1 mm for 14 numbers of channels and 1.2 mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979 mm² and 6171 m², while the inner surface areas are 13,545 mm² and 8856 mm² for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at the heat fluxes between 15 and 65 kW/m², mass flux of refrigerant between 300 and 800 kg/m² s and saturation pressure ranging from 4 to 6 bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed.